Precision non-contact three-dimensional (3D) metrology based on fringe interferometry has been developed for a range of industrial applications. Generally measurements are made over large volumes at low data acquisition speeds. For a variety of applications such as medical and dental imaging a 3D imaging system typically requires high spatial resolution and short measurement durations. Furthermore, many systems are limited in size to enable use by an operator such as a technician, doctor or dentist. PCT Patent Application No. PCT/US08/80940, incorporated by reference herein, describes examples of compact 3D imaging systems.
Conventional systems addressing the above limitations are limited in speed due to the requirement to accurately move (i.e., phase shift) fringes projected onto the surface of the object being measured. Some systems are based on moving a component in the fringe projector with submicron precision. Other systems require precision motion of a fringe projector in small steps. For example, U.S. Pat. No. 4,964,770 discloses a 3D measurement system which projects a pattern of stripes on the surface of a tooth. An image of the stripe pattern is acquired before the stripe projector is moved to other positions where additional images of the stripe pattern are acquired. Thus the system depends on precision motion of the stripe projector, projector pointing stability and the stability of the object (tooth) throughout the data acquisition interval.
Phase Measurement Interferometry (PMI) techniques are used in some precision non-contact 3D metrology systems where coherent radiation scattered from an object to be measured is combined with a coherent reference beam to generate an interference fringe pattern at a detector array. The phase of the reference beam is varied and images of the fringe patterns are acquired for multiple phase values.
Accordion Fringe Interferometry (AFI) techniques, as described, for example, in U.S. Pat. No. 5,870,191 and incorporated by reference herein, utilize a fringe projector that includes two closely-spaced coherent light sources to project an interferometric fringe pattern onto an object to be measured. Two (or more) precision shifts in the phase difference of the two light sources are made and at least one image of the fringe pattern is acquired for each of the three (or more) phase differences. Techniques used to achieve the phase shifts include precise translation of a diffraction grating and precise repositioning of an optical fiber. Both techniques rely on mechanical translation mechanisms capable of submicron positioning stability and systems employing these techniques are limited in data acquisition speed due to the settling time of the translation mechanisms.